Murine acquired immunodeficiency syndrome (MAIDS) is caused by a defective murine leukemia virus. The development of the MAIDS requires the presence of CD4 positive T cells and mature B cells. A helper free MAIDs virus which lacks the capacity to produce infectious virus in vivo was shown to be pathogenic. Furthermore, virus infected B cells stimulated syngeneic T cells in vitro. These results strongly suggest that the virus immune system interaction rather than the direct effect of virus infection play a key role in MAIDS. MAIDS is characterized by a development of lymphoadenopathy, polyclonal B cell activation, hypergammaglobulinemia and profound immunodeficiency in both T and B cells. Some of these symptoms are similar to those found in human Aids. Recent analysis demonstrated additional features shared by MAIDs and HIV infection which include involvement of superantigen, the cytokines produced by T cells and programmed lymphocytes death. These findings indicate that, despite the fundamental difference between two viruses, the mechanisms by which virus induces a variety of symptoms, especially immunodeficiency in T cells, may be shared. The long term goal of the study is to understand the MAIDS virus - immune system interaction in the development of the imunodeficiency. Understanding of the pathogenicity of the MAIDs virus will contribute to the understanding of general retrovirus induced diseases, including HIV infection in man.